The present invention relates generally to the mounting of optical elements and, more particularly, to apparatus and methods for establishing and maintaining a secure, aligned attachment between an optical element and a mounting plate of an optical system.
The performance of optical systems, such as a light engine for a conventional projection image display system, is sensitive to the mounting and alignment of the constituent optical elements. Typically, the optical system incorporates multiple optical components or elements that are each aligned with one or more optical axes. A positional misalignment of one optical element of only a few microns (xcexcm) or a rotational misalignment of one optical element by a few tenths of an angular degree suffices to degrade one or more attributes of the image transmitted by the optical system.
Once aligned, the optical elements of an optical system must be held securely to resist shock, vibration and thermal stress due to transit, storage and operating environment. Due to various deficiencies in conventional apparatus and methods for coupling optical elements with mounting plates or mounting pads on a mounting plate, the alignment of an optical element established during assembly of the optical system may be subsequently lost. Relatively large changes in temperature may cause a conventionally-mounted optical element to change its translational position and/or angular orientation with respect to its mounting plate and with respect to other optical elements in the optical system. Because the precise positioning and alignment of the optical elements is crucial to the effective performance of the optical system, any thermally-induced displacement or rotation of the individual optical elements degrades the performance of the optical system as a whole. In extreme instances, thermally-induced movement relative to the mounting plate can actually cause physical damage to the optical element.
One conventional mounting method entails adhesively bonding an optical element, typically glass, to a mounting pad on a mounting plate formed of, for example, a metal. Room temperature curing of the adhesive is a protracted process which may take as long as seventy-two hours to achieve a full cure. The curing may be significantly accelerated by heating the adhesive to an elevated temperature. Although the curing time is reduced by heating, the time for the adhesive to fully cure remains on the order of hours. Therefore, curing the adhesive coupling the optical element with the mounting plate slows the throughput of the assembly process for the optical system. As the curing temperature is increased to accelerate curing for speeding the assembly throughput, it becomes more likely that the optical element may be damaged by the heat treatment. In addition, the expense of the assembly process is significantly increased because curing ovens must be purchased to perform the heat treatment. Moreover, the curing ovens occupy valuable floorspace in the assembly area for the optical system.
To reduce the curing time, a radiation-curable adhesive may be used for adhesively bonding the optical element to the mounting pad on the mounting plate. Conventional radiation-curable adhesives are usually cured with radiation from the ultraviolet portion of the electromagnetic spectrum, which is transmitted through the optical element to radiate the underlying adhesive. However, transmitting the curing radiation through the optical element to the adhesive presents a significant risk of causing irreversible damage to the optical element from either heating or ultraviolet exposure.
Thus, there is a need for apparatus and methods for mounting an optical element to a mounting plate such that the optical element is secured to the mounting plate for resisting shock, vibration and thermal stress arising from transit, storage and operating environment and such that the mounting operation does not damage the optical element.
The present invention overcomes the foregoing and other shortcomings and drawbacks of mounting optical elements in an optical system, such as optical elements that are mounted to mounting plates as components of a light engine in a projection image display system. According to the principles of the invention, an optical assembly is provided that includes a mounting plate having a front surface, an opposed rear surface, and a throughhole extending between the rear surface and the front surface. The optical assembly further includes an optical element positionable adjacent to the front surface so as to block the throughhole, a radiation-transmissive member, such as a circular disk, obstructing at least a portion of the throughhole, and a first quantity of radiation-curable adhesive securing the radiation-transmissive member to the optical element. The first quantity of radiation-curable adhesive is capable of being cured by radiation incident from the rear surface of the mounting plate and transmitted through the radiation-transmissive member. The radiation used to cure the first quantity of radiation-curable adhesive is not transmitted through the optical element to effect curing.
In an alternative embodiment of the invention, the optical assembly may further include a second quantity of radiation-curable adhesive securing the radiation-transmissive member to the mounting plate. The second quantity of radiation-curable adhesive is capable of being cured by radiation incident from at least one of the rear surface or the front surface of the mounting plate. The radiation used to cure the second quantity of radiation-curable adhesive is not transmitted through the optical element to effect curing.
According to the principles of the invention, an optical assembly is provided that includes a mounting plate having a front surface, an rear surface opposite the front surface, and one circular throughhole and one oval throughhole each extending between the rear surface and the front surface. The oval throughhole has a major axis aligned substantially with a center of the circular throughhole and a minor axis orthogonal to the major axis. The optical assembly further includes an optical element coupled with the mounting plate such that the optical element is movable relative to the mounting plate. A first circular member is positioned within the circular throughhole and is adhesively secured to the optical element and to the mounting plate. Positioned within the oval throughhole is a second circular member which is adhesively secured only to the optical element. The second circular member has a diameter substantially equal to a width of the oval throughhole along the minor axis. The second circular member and the oval throughbore cooperate to constrain movement of the optical element to a direction substantially parallel to the major axis.
According to the principles of the invention, a method is provided for assembling an optical element with a mounting plate. The method includes providing the mounting plate with a throughhole, positioning the optical element adjacent to a front side of the mounting plate such that a portion of the optical element blocks an entrance to the throughhole, placing a radiation-transmissive member such that at least a portion of the throughhole is obstructed, supplying a first quantity of radiation-curable adhesive between the radiation-transmissive member and the optical element, and exposing the first quantity of radiation-curable adhesive to radiation from a rear side of the mounting plate through the radiation-transmissive member to irradiate the first quantity of radiation-curable adhesive. The radiation is effective to adhesively secure the radiation-transmissive member with the optical element.
In an alternative embodiment of the invention, the method may further include supplying a second quantity of radiation-curable adhesive between the radiation-transmissive member and the mounting plate and exposing the second quantity of radiation-curable adhesive to radiation effective to adhesively secure the radiation-transmissive member with the mounting plate.
The invention permits an optical element of an optical system, such as an optical element of a light engine for use in a projection screen display, to be precisely mounted and cured in position, thereby controlling the motion of the optical element during transit, storage and operation. In an optical system such as a light engine that combines multiple image components to form a composite image, the microscopic pixel size of the individual image components, which may vary considerably but may be as small as about 10 xcexcm, demands mounting techniques that maintain precise alignment during operation. A displacement in one of the individual image components by a distance equal to a fraction of the pixel size suffices to degrade the quality of the image output by the optical system.
According to the principles of the invention, adhesive couplings between the optical element and a mounting plate prevent misalignment and misorientation of the optical element during operation of the optical system resulting from, for example, differential thermal expansion arising from mismatches in coefficient of thermal expansion between the optical element and mounting plate. The flexibility of the adhesive layers between the optical element and the mounting plate accommodates the differential thermal expansion while maintaining the optical element substantially stationary and angularly oriented. As a result, the location of the optical element is controlled during operation of the optical system and the optical element is less likely to become misaligned or misoriented due to thermal effects. In addition, differential thermal expansion is significantly less likely to harm or otherwise damage the surface of the optical element. The principles of the invention are also effective for reducing the susceptibility of the mounted optical element to shock and vibration.
According to the principles of the invention, the curing time of the adhesive layers is significantly hastened by the use of radiation-curable adhesive. In addition, the use of a radiation-transmissive member permits direct irradiation of the radiation-curable adhesive in a direct optical path from the side of the mounting plate opposite to the optical element being adhesively secured thereto. As a result, the radiation-curable adhesive is not irradiated through the optical element being mounted, which significantly reduces or eliminates the risk of damage to the optical element from the curing radiation. Specifically, backside irradiation reduces the transfer of heat to the optical element and, in particular, curing with ultraviolet radiation from the backside is less likely to degrade the optical element. Moreover, the spatial tolerances and parallelism between at least the optical element and the radiation-transmissive member may be maintained by spacer elements in the radiation-curable adhesive.
The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof.